Milking apparatus for laboratory animals

ABSTRACT

A milking apparatus for laboratory animals enables a milking operation to be performed on a laboratory animal (such as a rat) by a single experimenter, and also enables universal milking data to be obtained that is not influenced by individual difference among experimenters. A teat cup enables milking to be performed directly from the nipple of a relatively small and prolific laboratory animal such as a rat and mouse. The invention provides useful means for clarifying the influence of harmful extrinsic substances on living organisms via milk. 
     The apparatus comprises a milk-collecting container  10  whose inside can be kept blocked from the outside air, two, i.e., a first and second, tubes  12, 13  inserted into the milk-collecting container in a communicated manner, and a teat cup  20  replaceably mounted on the other end of the first tube  12 . The other end of the second tube  13  is provided with a pressure switching means such as electromagnetic valves  33 A and  33 B that can switch the state inside the second tube  13  between a state where the inside is connected to the negative pressure creating source  35  and another state where it is opened to the atmosphere.

TECHNICAL FIELD

The present invention relates to a milking apparatus for laboratoryanimals, and more particularly to a milking apparatus suitable forrelatively small and prolific laboratory animals such as, e.g., rats andmice.

BACKGROUND ART

Recent years have seen the presence of substances responsible forenvironmental pollution and harmful to living bodies, such ascarcinogenic substances and endocrine disrupting (chemical) substances,becoming a global problem. It is of concern how these harmful substancesmay affect living bodies through milk such as breast milk. If it becomespossible to collect milk from laboratory animals such as rats and miceby a simpler method, such a method would be a useful means of clarifyingthe influence of the above-mentioned harmful extrinsic substances onliving bodies.

However, relatively small and prolific laboratory animals such as ratsand mice have a small nipple and produce only a small amount of milk,and for this reason there has not been proposed an effective milkingapparatus. For example, it is difficult to collect milk directly fromthe nipple of a rat. In laboratories, therefore, milk is usuallycollected from the stomach of a baby rat that has been fed with milk. Inthe milk collected from the stomach, however, the milk component maypotentially be broken down and digested by having been mixed with salivaand gastric juice and stirred. It is, therefore, an open questionwhether such milk can be called authentic milk.

C. T. Rodgers proposes (Laboratory Animals (1995) 29, 450-455) anapparatus for directly milking a rat, as shown in FIG. 11. In thisapparatus, a milk-collecting test tube 61 is arranged in a glasscontainer 60, which is then closed by a lid 62. One end of a flexibletube 63 is inserted into the milk-collecting test tube 61 in acommunicated manner. The flexible tube 63 extends through the lid 62 tothe outside of the glass container 60, and its front end is attached toa teat cup 64 made of silicon. The lower part of the glass container 60is provided with two openings 65, 66, to the latter of which is attacheda tube connected to a negative pressure creating source (not shown).

When a rat is milked by using this apparatus, vacuum is drawn from oneopening 66 by continuously operating the negative pressure creatingsource, while the experimenter closes and opens the other opening 65 bya finger to thereby produce a pulsation. The pulsation is transmitted tothe teat cup 64 via the flexible tube 63, and milk is collected by theteat cup 64 applied to the nipple of the rat. The above mentioned paperreports that 1.0-1.5 ml of milk was collected, by using the aboveapparatus, from all the nipples of mother rats on the fourteenth day oflactation.

The above milking apparatus shows that it is possible to directly milkfrom the nipple of a rat and the apparatus is therefore useful. Thismilking apparatus, however, requires two experimenters to operate.Namely, one experimenter must put the mother rat in place by one handand operate the teat cup 64 by the other. The other experimenter mustopen and close the front end of the pressure-reducing opening 65 withhis or her index finger in order to produce a pulsation. Further, sincethe beat is produced by the finger operation by the experimenter, errorstend to arise due to differences between individual experimenters. Thus,it is not easy to obtain data such as universal milked amount. Moreover,there has been no concrete report about the shape of the teat cup, andthis remains an issue to be addressed in the future for better milking.

Accordingly, it is an object of the present invention to provide a novelmilking apparatus for laboratory animals by which a single experimentercan perform a milking operation and which makes it possible to obtainmilking data universally not influenced by individual differences amongexperimenters. It is another object of the present invention to providea novel teat cup particularly suitable for a milking apparatus forlaboratory animals.

DISCLOSURE OF THE INVENTION

The present invention provides a milking apparatus for laboratoryanimals comprising a milk-collecting container whose inside can be keptshut from the outside air, a first and second tubes having one of theirends inserted into an upper part of the milk-collecting container insuch a manner as to communicate with the inside of the container, a teatcup replaceably attached to the other end of the first tube, a negativepressure creating source attached to other end of the second tube, and apressure switching means for switching, in a pulsed manner, the stateinside of the second tube between an atmospheric pressure state and anegative pressure state created by the negative pressure creatingsource.

In this milking apparatus for laboratory animals, the negative pressurecreating source is continuously operated, and a negative pressure isdeveloped inside the second tube by connecting it with the negativepressure creating source by means of the pressure switching means. Thenegative pressure is directly transmitted from the milk-collectingcontainer to the teat cup via the first tube. As the pressure switchingmeans opens the second tube to the atmosphere and the atmosphericpressure develops therein, the teat cup is brought under the atmosphericpressure. Thus, by operating the pressure switching means in a pulsatingor cyclical manner, a continuous pulsation develops at the front end ofthe teat cup.

In that state, the experimenter can perform a required milking operationby putting the mother rat in place with one hand while applying the teatcup to the nipple of the laboratory animal with the other hand. Thismeans that a continuous milking operation can be performed by a singleexperimenter. This is a great advantage provided by the presentinvention. Since the repetition of the negative pressure and theatmospheric pressure directly acts on the nipple of the laboratoryanimal via the teat cup, the milking operation can smoothly proceed evenin the case of a small laboratory animal such as a rat.

The milk obtained from the laboratory animal by the pulsation of theteat cup is collected at the lower portion of the milk-collectingcontainer via the first tube. For structural reasons, the milk collectedin the milk-collecting container does not flow into the second tube,thereby preventing the pressure switching means and the negativepressure-creating source from being subjected to operational failure.

In the milking apparatus for laboratory animals according to the presentinvention, the negative pressure creating source may be of any type. Ina preferred embodiment, however, a vacuum pump and a pressure controllerfor controlling the pressure inside the second tube are used as a singleunit in constructing the milking apparatus for laboratory animals. Byusing a negative pressure tank such as an accumulator, a stablepulsation can be obtained. Alternatively, a negative pressure creatingmeans using, as the operating source, the amount of motion of a fluid(such as tap water, air, vapour and the like) flowing at a substantiallystable flow rate, such as an aspirator, may be used. In this case, thenegative pressure can be easily adjusted to a desired value by adjustingthe flow rate of the fluid by opening or closing the tap without usingthe pressure controller, thereby simplifying the apparatus.

The pressure switching means may comprise an open/close valve such as atwo-way electromagnetic valve capable of switching the state inside thesecond tube between a state where the inside is opened to the atmosphereand a state where the inside is blocked therefrom, with the second tubebeing connected to the negative pressure source at all times. In anotherembodiment, the pressure switching means may comprise a first open/closevalve for connecting and disconnecting the second tube to and from thenegative pressure source and a second open/close valve for switching thestate inside the second tube upstream of the first open/close valvebetween a state where the inside is opened to the atmosphere and anotherstate where the inside is blocked therefrom. In the latter case, too, anopen/close valve such as a two-way electromagnetic valve is preferablyused. In a further embodiment, an open/close valve of the pulsator valvetype conventionally used in cow milking apparatus may be advantageouslyused.

In the case of using an electromagnetic valve, by providing acomputer-controlled control means for controlling the open/close timingof each electromagnetic valve, a stable milking operation can beperformed, and this also makes it possible to milk a variety oflaboratory animals with the same apparatus under optimum conditions. Inaddition, in the present invention, the teat cup is adapted to bereplaceably mounted on one end of the first tube, so that a teat cupwith an optimum shape for a given laboratory animal to be milked can beeasily selected and used.

Preferably, the teat cup comprises a top surface portion (liner) havingan insertion hole into which the nipple of the laboratory animal is tobe inserted, and an outer fitting portion to be fitted with the firsttube, the outer fitting portion extending from the periphery of the topsurface portion. At least those portions of the top surface portion nearthe insertion hole are provided with softness and flexibility such that,during milking, they can deform inwardly when a negative pressuredevelops inside the first tube and return to their original shape whenthe atmospheric pressure is returned therein.

In the teat cup of this shape, the insertion hole deforms when the topsurface portion (i.e., the portion that comes into direct contact withthe nipple and udder during milking) deforms inwardly when at negativepressure (milking period), when milk is sucked from the nipple. When atatmospheric pressure (resting period), it returns to its originalposture. Such deformation of the top surface portion can provide thesame effect as that of stimulating the nipple of the mother by theoffspring of the laboratory animals, so that an effective milkingoperation can proceed.

Experiments show that the above effect can be enhanced by selectively oradditionally adopting the features of: making an internalcircumferential surface of the insertion hole of the teat cup inclinedsuch that the diameter of the insertion hole becomes smaller towards thefirst tube with which the teat cup is fitted; forming cuts radially onthe teat cup near the insertion hole; and providing a front-end portionof the first tube where the teat cup is mounted with an inclination suchthat the diameter of the first tube becomes greater towards the teat cupside.

In the experiments conducted by the inventors, in the case of a rat as alaboratory animal, a maximum amount of milk was obtained when a negativepressure of about −210 mmHg, preferably about −150 mmHg, was created atthe front end of the teat cup for a predetermined period of time duringmilking (suction period), though depending on the level of pressure atwhich the pressure switching means can operate stably. Thus, in themilking apparatus for laboratory animals according to the presentinvention, preferably a negative pressure in the range of from 0 mmHg to−210 mmHg, preferably from 0 mmHg to −150 mmHg, is created at the frontend of the teat cup during milking.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-4 show different embodiments of the milking apparatus forlaboratory animals according to the present invention adapted formilking rats.

FIG. 5 shows a perspective view for the explanation of an embodiment ofthe teat cup according to the present invention as attached to a tube.

FIG. 6. shows a sectional view taken along the line a—a of FIG. 5.

FIGS. 7 and 8 show sectional views of different examples of the mannerin which the teat cup is attached to a first tube.

FIG. 9 shows a pulsation characteristics curve (pulsogram) of themilking apparatus for rats shown in FIG. 1.

FIG. 10 shows a pulsation characteristics curve (pulsogram) of themilking apparatus for rats shown in FIG. 4.

FIG. 11 is a drawing for the explanation of a milking apparatus for ratsproposed by the prior art.

In the drawings: 10 test tube to be used as a milk-collecting container11 sealing plug 12 first tube 13 second tube 20 teat cup 31 backflowprevention valve 32 vacuum meter 33 sliding-valve type pulsator valve33A, 51, 55 electromagnetic valve 34 filtering jar to be used as apressure-control tank 35 vacuum pump with a pressure-control valve 36filtering jar to be used as a silencing means 40 aspirator to be used asa negative-pressure source 50 electromagnetic valve open/close controlmeans (computer)

BEST MODE OF CARRYING OUT THE INVENTION

Hereafter several embodiments of the milking apparatus for laboratoryanimals according to the present invention will be described. FIG. 1shows an example of the milking apparatus for laboratory animalsaccording to the present invention which is assembled as a milkingapparatus for rats. A sealing plug 11 is attached to a test tube 10which functions as a milk-collecting container. A first tube 12 and asecond tube 13 are connected via glass tubes inserted in the sealingplug 11 in such a manner as to communicate with the inside of the testtube 10. In this example, the first tube 12 and the second tube 13 areformed by plastic tubes, but they may be silicon tubes. The other end ofthe first tube 12 is attached to a teat cup 20 in a replaceable manner.This cup will be described later.

The other end of the second tube 13 is connected to one side of asliding-valve type pulsator valve 33 via a backflow prevention valve 31made of hard glass and a vacuum meter 32. The other side of the pulsatorvalve 33 is connected, via a filtering jar 34 which functions as apressure-control tank or an accumulator, to a vacuum pump 35 with apressure-control valve. Further, in order to eliminate the operatingnoise of the vacuum pump 35, the exhaust side of the vacuum pump isconnected to an intake opening of a filtering jar 36. The filtering jar36 is connected to an exhaust pipe 38 via an L-shaped tube 37 made ofglass. Such silencing means is often required when milking a smalllaboratory animal such as a rat without anesthesia, so as not to excitethe animal. These connections between devices are preferably made by asilicon tube 14.

In the rat milking apparatus with the above structure, after suitablyadjusting the vacuum pump 35 and the pulsator valve 33 connected to thevacuum pump such that the negative pressure suitable for the milking ofthe rat and the atmospheric pressure pulsate at uniform intervals, theexperimenter holds the rat with one hand while milking the rat byapplying the front end of the teat cup 20 onto the nipple of the ratwith the other hand. This milking operation can be entirely done by asingle experimenter.

FIG. 2 shows another embodiment of the milking apparatus for laboratoryanimals according to the present invention. This embodiment differs fromthe apparatus shown in FIG. 1 in that, instead of the vacuum pump 35 asthe negative pressure creating source, a negative pressure creatingmeans (aspirator) 40 is employed. The aspirator 40 employs, as a sourceof operation, the amount of motion of a fluid (such as tap water) whichflows substantially at a uniform flow rate. Further, instead of thepulsator 33, a two-way electromagnetic valve 33A whose opening andclosing is controlled by a control means 50 using a computer (thecontrol means 50 may be one well known in the art and so its descriptionis omitted). The above-mentioned other end of the second tube 13 isconnected, via the backflow prevention valve 31 and vacuum controller39, to an intake side of the aspirator 40. The electromagnetic valve 33Ais disposed between the vacuum controller 39 and the aspirator 40, suchthat the inside of the second tube 13 and device-connecting tube 14 canbe switched between a state where it is connected to the aspirator 40and another state where it is opened to the atmosphere.

The aspirator 40 is as known in the art and equipped with a conduit pipe41 and a drainage pipe 43 having an expanded portion 42 at the front endfor accommodating an ending 41 a of the conduit pipe 41. Negativepressure is created within the expanded portion 42 by causing nozzlesuction between a reduced-diameter portion 44 of the expanded portion 42of the drainage pipe 43 and the ending 41 a of the conduit pipe 41. Theinside of the expanded portion 42 is connected, via a branch pipe 45formed thereon and the backflow prevention valve 46, to the second tube13 (connecting tube 14).

Since this embodiment does not employ a vacuum pump, the apparatusgenerates less noise and can be made smaller in size. By adjusting thevacuum controller 39, a desired negative pressure can be easilyobtained. When, as shown, tap water is used as the fluid for theoperation of the aspirator 40 in a laboratory, for example, the negativepressure can be easily controlled by adjusting the degree of opening ofa tap 47 as well. Further, the adoption of the electromagnetic valve 33Aand the control means 50 for controlling the opening and closing of thevalve enables a pulsation with desired intervals to be obtained easilyand thus makes it easier to deal with a variety of laboratory animals inan appropriate manner.

FIG. 3 shows another embodiment of the milking apparatus for laboratoryanimals according to the present invention. In this example, the vacuumcontroller 39 is removed from the apparatus of FIG. 2 and there is onlythe vacuum meter 32. In this case, too, any desired negative pressurenecessary for milking experiments can be easily obtained by controllingthe degree of opening of the tap 47 connected to the aspirator 40.

It is of course possible to replace, in the milking apparatus of FIG. 1,the pulsator 33 with an electromagnetic valve and the control means 50for controlling the valve's opening and closing as shown in FIGS. 2 and3. FIG. 4 schematically shows an example of the milking apparatus ofthat type, in which a first branch pipe 14 a has a pressure sensor 51while a second branch pipe 14 b provided further downstream has a firsttwo-way electromagnetic valve 33A. The first two-way electromagneticvalve 33A can be opened to open the second branch pipe 14 b to theatmosphere. A second tube 13 has a second two-way electromagnetic valve33B downstream of the second branch pipe 14 b. By opening this valve,the other end of the second tube 13 can be communicated to a negativepressure tank 34 a which functions as a pressure-control tank or anaccumulator. The negative-pressure tank 34 a is connected to the vacuumpump 35 and maintained at a negative pressure.

A computer 50 is connected such that information can be transmittedamong the pressure sensor 51, the first and second electromagneticvalves 33A, 33B, and the vacuum pump 35. The computer 50 controls theinput of pressure information from the pressure sensor 51, controloutput for the opening and closing timings of the first and secondelectromagnetic valves 33A, 33B, and operation control output for thevacuum pump 35.

An example of the operation of the milking apparatus will now bedescribed. The vacuum pump 35 is operated while the first and secondelectromagnetic valves 33A, 33B are closed, such that a negativepressure is created in the negative pressure tank 34 a. After apredetermined pressure is established, the second electromagnetic valve33B is opened. As a result, the air inside the second tube 13 is drawninto the negative pressure tank 34 a, thereby creating a predeterminednegative pressure (set pressure) with respect to the atmosphericpressure. At that point in time, the second electromagnetic valve 33B isclosed, thereby establishing and maintaining the set negative pressurestate in the second tube 13 (i.e., suction from the teat cup isperformed).

The above state is maintained for a predetermined period of time andthen the first electromagnetic valve 33A is opened. This opens theinside of the second tube 13 to the atmosphere, so that the pressurereturns to the atmospheric pressure. After maintaining this state for apredetermined period of time, the second electromagnetic valve 33B isagain opened. Thereafter, this cycle is repeated a set number of times,thus proceeding with a desired milking operation. If, during the suctionstep, a minimum value of the set negative pressure is reached, thesecond electromagnetic valve 33B is temporarily opened to therebyre-suck and bring the pressure back to the set value.

Next, the teat cup used in the above apparatus will be described indetail. FIG. 5 shows a perspective view of an embodiment of the teat cup20, which is connected to the front end of the first tube 12. FIG. 6shows a sectional view taken along the line a—a of FIG. 5. In thisexample, the first tube 12 is about 2 mm in internal diameter and about4 mm in external diameter. A silicon O-ring 15 (about 2 mm in internaldiameter and about 1 mm in thickness) is fixed with glue at about 2 mmfrom the front end of the first tube 12, for the purpose of positioningthe teat cup 20. The teat cup 20 is made of soft rubber (such as siliconrubber) and generally cylindrically shaped with a front end closed by atop surface portion (liner) 21 a. It has an internal diameter d=about 4mm, an external diameter D=about 7 mm, and a height h=about 4 mm. Inthis example, as shown, a gap a of about 1.6 mm is formed between theback surface of the top surface portion 21 a and the front end of thefirst tube 12 by inserting the front end of the first tube 12 into anouter fitting portion 21 b of the teat cup 20.

The top surface portion 21 a of the teat cup 20 functions as a linerwith which the nipple and udder of a mother rat come into direct contactand has a thickness b=about 0.4 mm. A center portion of the teat cup 20has an insertion hole 22 with a diameter c1=about 1.8 mm into which thenipple of the rat is to be inserted. The insertion hole 22 is about 0.8mm in length t and is conically shaped with an enlarged diameter on itsfront-end side. Its rear-end diameter c2=about 1.4 mm. Further, the topsurface portion 21 a has six cuts 25 each extending radially from aninner circumferential edge of the insertion hole 22 and measuring about1.3 mm in length.

In this teat cup 20, the insertion hole 22 opens up greatly during amilking period (reached vacuum degree period, suction period), asillustrated by the phantom line in FIG. 6, thereby sucking milk from thenipple. The sucked milk is stored in the test tube 10 via the first tube12. During a resting period (a reached atmospheric pressure period, whenthe above-mentioned negative pressure is not present, an atmosphericpressure period/massaging period), the insertion hole 22 formed at thetop surface portion 21 a closes by its own resilience (i.e., back to theoriginal posture as shown by the solid line in FIG. 6). As mentionedabove, by this opening and closing action of the insertion hole 22, aneffect similar to that of a baby rat sucking milk and thus stimulatingthe nipple and udder of the mother rat can be obtained. Thus, the actionof the insertion hole 22 is very important.

FIG. 7 shows a sectional view of another example of the manner in whichthe teat cup 20 is attached to the first tube 12. Here, the teat cup 20is similar to that shown in FIGS. 5 and 6 except that a front-endportion of the first tube 12 to which the teat cup is to be attached isformed with an inclined surface 12 a (preferably with an inclination ofabout 15°) such that the diameter of the first tube 12 becomes largertowards the side of the teat cup 20. The thus inclined front-end portionreaches as far as the back surface of the top surface portion 21 a ofthe teat cup 20, in contrast to the example of FIG. 6. In this case,too, since the inclined surface 12 a is formed at the front end of thefist tube 12, the opening and closing action of the insertion hole 22proceeds in the same manner as that shown in FIGS. 5 and 6. Since thisexample does not require the O-ring 15, the structure can beadvantageously simplified.

FIG. 8 shows a sectional view of another example of the manner in whichthe teat cup is attached to the first tube 12. Here, the insertion hole22A formed in the teat cup 20A has a length t which is equal to thethickness b=0.4 mm of the top surface portion 21 a. The teat cup has adiameter c1 of 1.8 mm at its front-end side and a diameter c2a of 1.5 mmat its rear-end side, and is thus conically shaped. Experiments showedthat the teat cup 20A of this type, when used in combination with thefirst tube formed at the front end with the inclined surface 12 a, wascapable of providing the same opening and closing action of theinsertion hole. Though not shown, the experiments also confirmed thatalmost identical opening and closing action can be obtained in this casewithout providing the six cuts 25. Presumably this is due to the resultof the top surface portion 21 of the teat cup 20A extending andcontracting circumferentially as well.

Hereafter, an experiment is described in which a rat was milked bymounting the teat cup 20 shown in FIGS. 5 and 6 on the rat milkingapparatus shown in FIG. 1. FIG. 9 shows a pulse characteristics curve(pulsogram) of the present experiment, and Table 1 shows the analysisvalues. When the analysis results of the pulsogram in the above ratmilking apparatus were compared with those of the pulsogram in a milkingapparatus for cows which is not shown, the values were withininternational standard values for cow milking apparatus, except for thedegree of vacuum. A standard degree of vacuum in a cow milking apparatusis about −330 mmHg, in the range of from −300 mmHg to −350 mmHg. In theabove milking apparatus for rats, the greatest amount of milk wasobtained when the degree of vacuum was about −210 mmHg and under theconditions shown in Table 1. In FIG. 9 and Table 1, A: Transitionalperiod (reached vacuum degree period), B: Suction period, C:Transitional period (reached atmospheric pressure period), and D:Atmospheric pressure period. A+B indicates the milking period, and C+Dindicates the resting period (massaging period).

TABLE 1 Degree of vacuum (-mmHg) 210 Number of pulses (min) ^(a)) 46Pulsation ratio (suction period 66 ratio) ^(b)) A (msec) 140 B (msec)720 C (msec) 80 D (msec) 360 A + B (msec) 860 C + D (msec) 440 ^(a))60/A + B + C + D × 1000 = times/min ^(b)) A + B/A + B + C + D × 100

The details of the experiment are as follows. Eight-week-oldSprague-Dawley rats (Jcl: SD, Clea Japan Inc.) were raised underconditions of 40 to 60% humidity, 23±1° C. temperature, and 14-hoursillumination (from 5 a.m. to 7 p.m.). Feed (CE-2Clea Japan Inc.) andwater were freely provided. After three weeks of acclimation under theseconditions, only visibly healthy rats were selected and used in theexperiment. A 12-week-old female rat regularly exhibiting a sexual cycleand a male rat of similar variety were put in an aluminum cage andmated. Pregnant rats were transferred to a polyacrylic cage fordelivery. Four days after delivery, the number of offsprings wasrandomly adjusted to four males and four females, totalling eightoffsprings.

The eight offsprings were separated from their mothers at 8 a.m., 14days after delivery when the rats lactate the most. At 4 p.m., themothers were given a hypodermic injection of Oxytocin 1 IU (Atonin-O,Teikoku Zoki Co.,). A method of measuring the amount of milk based onweight differences in the mother rat and baby rat before and afterbreast-feeding was used. Twenty minutes after the injection of Oxytocin,the mother rat was anesthetized (sodium pentobarbital), and the nipple,udder and their vicinity were sterilized by 70%-alcohol cotton. Aftergently massaging the mammary gland with a finger, the top surfaceportion 21 a of the teat cup 20 was made, by using one hand, to suck thenipple and milking was initiated. During the milking operation, themammary gland was continuously gently massaged by the thumb and indexfinger of the other hand. Milking was possible from all of the nipplesfrom which the eight offsprings were fed.

Table 2 shows the amounts of milk milked from all of the nipples fromwhich the eight offsprings were fed, under the conditions of Table 1. Anaverage amount of milk obtained from a mother rat on the fourteenth dayof lactation was 3.43±1.71 g. This amount is less than the milked amountcalculated from the weight differences in the mother rat and baby ratbefore and after breast-feeding, but is sufficient for today's chemicalanalysis methods.

TABLE 2 Milked amount (g/mother rat) Number of mother rats 10 Average ±standard deviation 3.43 ± 1.71 Range 1.4-6.5

Similarly, another rat milking experiment was conducted by attaching theteat cup 20 shown in FIGS. 2 and 3 to the rat milking apparatus shown inFIG. 4 in the following manner. FIG. 10 shows a pulse characteristicscurve (pulsogram) in the experiment, and Table 3 shows the analysisvalues. In the above rat milking apparatus, the greatest amount of milkwas obtained when the degree of vacuum was about −140 mmHg and under theconditions of Table 3. While the greatest amount of milk was obtained ata lower degree of vacuum than in the case of the rat milking apparatusshown in FIG. 1, presumably this is due to the fact that the apparatusof FIG. 1 required a higher degree of vacuum to facilitate a properoperation of the valve necessitated by the structure of the valve. InFIG. 10 and Table 3, A: Transitional period (reached vacuum degreeperiod)(first electromagnetic valve 33A closed, second electromagneticvalve 33B open), B: Suction period (first electromagnetic valve 33Aclosed, second electromagnetic valve 33 open), C: Transitional period(reached atmospheric pressure period) (first electromagnetic valve 33Aopen, second electromagnetic valve 33B closed), D: Atmospheric pressureperiod (first electromagnetic valve 33A closed, second electromagneticvalve 33B closed). A+B indicates a milking period, and C+D indicates aresting period (massaging period).

TABLE 3 Degree of vacuum (-mmHg) 140 Number of pulses ^(a)) 60 Ratio ofpulsation ^(b)) 60 A (msec) 100 B (msec) 500 C (msec) 80 D (msec) 320A + B (msec) 600 C + D (msec) 400 ^(a)) 60/A + B + C + D × 1000 =times/min ^(b)) A + B/A + B + C + D × 100

The details of the experiment were as follows. Eight-week-oldSprague-Dawley rats (Jcl:SD, Clea Japan Inc.) were raised under theconditions of 40-60% humidity, 23±1° C. temperature and 14 hoursillumination (5 a.m. to 7 p.m.). Feed (CE-2Clea Japan Inc.) and waterwere freely provided. After three weeks of acclimation under theseconditions, only visibly healthy rats were selected and used for theexperiment. Twelve-week-old female rats that regularly exhibited asexual cycle and male rats of similar variety were put in an aluminumcage and mated. Pregnant rats were transferred to a polyacrylic cage fordelivery. Four days after delivery, the number of baby rats was randomlyadjusted to four males and four females, totalling eight offsprings.

The eight offsprings were separated from the mothers at 8 a.m. 4, 7, 10,14, and 18 days after delivery, and the mothers were given hypodermicinjection of Oxytocin 1 IU (Atonin-O, Teikoku Zoki Co.,) at 4 p.m. Amethod of measuring the amount of milk based on weight differences inthe mother rat and baby rat before and after breast-feeding wasemployed. Twenty minutes after the Oxytocin injection, the mother ratswere anesthetized (sodium pentobarbital), and the nipple, udder andtheir vicinities were sterilized by 70%-alcohol cotton. After gentlymassaging the mammary gland by hand, the top surface portion 21 a of theteat cup 20 was made to suck the nipple by one hand, and milking wasinitiated. During milking, the udder was continuously and gentlymassaged by the thumb and index finger of the other hand. Milking waspossible from all the nipples from which the eight offsprings were fed.

Table 4 shows the amounts of milk obtained from all the nipples fromwhich the eight baby rats were fed, under the conditions shown in Table3. The average amount of milk obtained from the mother rats on thefourteenth day of lactation was 3.99±1.22 g, the highest amount fromamong the milking groups. While this amount was less than the amount ofmilk calculated from the weight differences in the mother rat and babyrat before and after breast-feeding, this amount is sufficient fortoday's chemical analysis methods.

TABLE 4 Number of days after delivery 4 7 10 14 18 Average 0.76 ± 0.481.66 ± 0.65 2.47 ± 0.67 3.99 ± 1.22 2.10 ± 0.93 milked amount g^(a))Range 0.28-1.78 1.01-2.88 1.59-3.57 2.05-6.21 0.28-3.27 Number of66(10/15^(b))) 100(15/15^(b))) 100(15/15^(b))) 100(15/15^(b)))80(12/15^(b))) mother rats with a milked amount 0.5 g or more (%) Thevalues are an average milked amount ± standard deviation of 15 motherrats. ^(a))Milked amount from all of the nipples ^(b))Number of motherrats with a milked amount of 0.5 g or more/number of all the mother ratsmilked × 100%

Thus, by using the milking apparatus for laboratory animals according tothe present invention, a single experimenter can perform a milkingoperation on a laboratory animal, and universal milking data that is notinfluenced by individual differences among experimenters can beobtained. By using the teat cup according to the present invention whichis particularly suitable for the milking apparatus for laboratoryanimals, relatively small and prolific laboratory animals such as ratsand mice can be directly milked from the nipple. Accordingly, thepresent invention can provide a useful means for revealing theinfluences of harmful extrinsic substances on the living bodies throughmilk.

What is claimed is:
 1. A milking apparatus for laboratory animals,comprising: a milk-collecting container whose inside can be kept shutfrom the outside air; first and second tubes having one of their endsinserted into an upper part of the milk-collecting container in such amanner as to communicate with the inside of the container; a teat cupreplaceably attached to the other end of the first tube; a negativepressure creating source attached to the other end of the second tube;and a pressure switching means disposed between the milk-collectingcontainer and the negative pressure creating source for switching, in apulsed manner, the state inside of the second tube between anatmospheric pressure state and a negative pressure state created by thenegative pressure creating source.
 2. A milking apparatus for laboratoryanimals according to claim 1, wherein the negative pressure creatingsource is a vacuum pump, the apparatus further comprising a pressurecontroller for controlling the pressure inside the second tube.
 3. Amilking apparatus for laboratory animals according to claim 1, whereinthe negative pressure creating source is a negative pressure creatingmeans having, as an operation source, the amount of motion of a fluidflowing at a substantially stable flow rate.
 4. A milking apparatus forlaboratory animals according to claim 1, wherein the pressure switchingmeans, with the second tube connected to the negative pressure creatingsource at all times, comprises an open/close valve capable of switchingthe state of the inside of the second tube between a state where theinside is opened to the atmosphere and another state where the inside isblocked from the atmosphere.
 5. A milking apparatus for laboratoryanimals according to claim 1, wherein the pressure switching meanscomprises a first open/close valve for connecting and disconnecting thesecond tube to and from the negative pressure creating source, and asecond open/close valve for switching the state inside the second tubeupstream of the first open/close valve between a state where the insideis opened to the atmosphere and another state where the inside isblocked from the atmosphere.
 6. A milking apparatus for laboratoryanimals according to claim 4, wherein the open/close valve is anelectromagnetic valve, the apparatus further comprising a control meansfor controlling the open/close timing of the electromagnetic valve.
 7. Amilking apparatus for laboratory animals according to claim 1, whereinthe pressure switching means includes a pulsator valve.
 8. A milkingapparatus for laboratory animals according to claim 1, wherein the teatcup comprises a top surface portion having an insertion hole into whichthe nipple of a laboratory animal is to be inserted, and an outerfitting portion extending from the periphery of the top surface portionfor fitting with the first tube, wherein at least portions of the topsurface portion near the insertion hole are provided with softness andflexibility such that they can, at the time of milking, deform inwardlywhen a negative pressure develops inside the first tube and return totheir original shape when the atmospheric pressure is present inside thefirst tube.
 9. A milking apparatus for laboratory animals according toclaim 8, wherein an internal peripheral surface of the insertion hole ofthe teat cup is inclined such that the diameter of the insertion hole issmaller towards the first tube.
 10. A milking apparatus for laboratoryanimals according to claim 8, wherein cuts are formed radially on thetop surface of the teat cup near the insertion hole.
 11. A milkingapparatus for laboratory animals according to claim 8, wherein afront-end portion of the first tube on which the teat cup is to bemounted is formed with an inclined surface such that the diameter of thefirst tube becomes larger towards the teat cup.
 12. A milking apparatusfor laboratory animals according to claim 1, wherein the apparatus isadapted for rats or mice.
 13. A milking apparatus for laboratory animalsaccording to claim 12, wherein a negative pressure created at the frontend of the teat cup during milking is in the range of from 0 mmHg to−210 mmHg, preferably from 0 mmHg to −150 mmHg.